Naive B cells migrate through the circulation to lymphoid organs where they undergo the T cell-dependent germinal center reaction. Adaptive immunity is acquired as specific antigen-reactive germinal center B cells differentiate into the major effector B cells of the adaptive immune system: memory cells and plasma cells. See, e.g., FIG. 1A. Although the role of specific transcription factors in mature B cell differentiation has been examined (see Nutt et al. Nature. 1999; 401:556-562; Chang et al. Proc Natl Acad Sci USA. 1996; 93:6947-6952; Turner et al. Cell. 1994; 77:297-306; Shaffer et al., Immunity. 2004; 21:81-93; and Schebesta et al. Curr Opin Immunol. 2002; 14:216-223), mechanisms regulating such transcription factors during mature B cell differentiation are largely unknown.
Many malignancies derived from mature B cells are known and are believed to constitute the majority of leukemias and lymphomas. Such malignancies appear to reflect defined stages of normal B cell differentiation. Diagnosis of leukemias and lymphomas can be particularly difficult because of their shared lineage. These cancers frequently display overlapping morphologies, genetic abnormalities, and expression of surface markers, which can complicate the diagnosis. However, the distinction of these tumors is clinically important because there are important differences in the treatments and expected response to treatment. Thus methods that improve the accuracy of their diagnosis should provide to improved outcomes for these patients.
MicroRNAs are commonly 18-22 nucleotide-long RNA molecules that regulate expression of genes. There is an increasing recognition of the role of microRNAs in oncogenesis, lineage-selection, and immune cell function, including early B cell differentiation. See Calin et al. N Engl J Med. 2005; 353:1793-1801; O'Donnell et al. Nature. 2005; 435:839-843; Chen et al. Science. 2004; 303:83-86; Lim et al. Nature. 2005; 433:769-773; Li et al. Cell. 2007; 129:147-161; Xiao et al., Nat Immunol. 2008; 9:405-414; Baltimore et al. Nat Immunol. 2008; 9:839-845; and Ventura et al. Cell. 2008; 132:875-886. However, the full extent and function of microRNA expression during mature B cell differentiation and in B cell malignancies are not known.
Correct diagnosis of B cell malignancies is important from both a clinical standpoint and from the standpoint of setting appropriate patient expectations. A misdiagnosed B cell malignancy may lead to an inappropriate therapy, which can unnecessarily endanger the patient's life and/or be an ineffective treatment for the B cell malignancy. As an example, the diagnostic distinction of Burkitt lymphoma (BL) from diffuse large B cell lymphomas (DLBCLs) can be difficult because of overlapping morphology, immunophenotype and cytogenetics. Burkitt lymphoma tumors are molecularly distinct from DLBCL, however. The difficulty and importance of obtaining the correct diagnosis in BL was highlighted by the experience of a multicenter clinical trial, CALGB trial#925119, in which nearly half of the 100 patients with an assigned diagnosis of BL were found to have another diagnosis upon further pathology review.
If diagnosed and treated appropriately, nearly 80% of patients with BL can be cured with intensive (high dose) chemotherapy regimens. Thus, a misclassification of BL as DLBCL can result in a missed opportunity to cure the malignancy. On the other hand, misclassification of DLBCL as BL leads to unnecessarily morbidity from intensive chemotherapy regimens. Thus, methods that improve the diagnosis of BL, and other B cell malignancies, can provide better outcomes in patients.
DLBCLs can also be subclassified into two different B cell malignancies, activated B-cell (ABC) DLBCL and germinal center B cell like (GCB) DLBCL. There are at least two important clinical applications for the molecular sub-grouping of DLBCL patients. First, the prognostic information could inform the choices and expectations of patients and their physicians. Second, the important molecular differences in these subgroups form the basis of testing different targeted therapies in these patients. The possibility of a differential response to therapy among ABC and GCB DLBCLs is supported by data that suggest that the benefit of receiving a proteosome inhibitor, bortozemib, is predominantly limited to those patients who have ABC DLBCL. However, the clinical distinction of the subgroups of DLBCL using immunohistochemistry is difficult with current methods distinguishing GCB DLBCL from non-GCB DLBCL with limited success.
Mature B cell differentiation is important for the development of adaptive immunity. The process is also of interest because B cell malignancies are common and retain a number of features derived from their normal counterpart B cell subsets. Unlike other maturation pathways in the hematopoietic and other cell lineages, successive stages of mature B cells do not simply signify progressive differentiation away from the stem cell stage. Rather, each stage represents a specialized state with specific functions. Thus, germinal center (GC) cells interact with CD4 T cells and dendritic cells and undergo somatic hypermutation and Ig-heavy chain class-switching. On the other hand, plasma cells secrete immunoglobulin, while memory cells are primed to proliferate and differentiate into plasma cells upon repeat contact with antigen. The specialized functions demand a finely tuned program of gene regulation.
MicroRNAs represent a novel class of biomarkers that provide new opportunities for clinical translation. First, intact microRNAs can be isolated from tissues preserved using standard methods, such as formalin fixed, paraffin embedded (FFPE) tissue. Thus, microRNA-based biomarkers could be easy to translate to clinical use. Second, microRNAs can be readily assayed using real-time PCR and other methods available in conventional pathology.